1. Field of the Invention
The present invention relates generally to an optical disc driving apparatus for processing a signal obtained from the optical disc so as to reproduce the information recorded thereon, and more particularly to access control of an optical head at the optical disc driving apparatus.
2. Description of the Prior Art
In the conventional optical disc driving apparatus, a push pull method using a pair of photo detectors is well-known as the method to detect a track error. This method comprises a tracking sensor signal which is the sensor signal of the tracking servo, obtained from a difference signal between the divided photo detectors and a radio frequency (RF) signal used for data demodulation or the like obtained from a sum signal thereof. The speed of the optical disc is detected from a cycle period of a sinewave like tracking sensor signal or the RF signal when the light spot traverses a guide groove on the optical disc, thereby performing the access control. The method is disclosed in, for example, the Japanese Patent Application Laid Open Gazette No. 61-260474 (1986).
With this method, however, if the information is recorded on a guide groove or between the guide grooves, the tracking sensor signal and RF signal are subjected to modulation by the recorded information signal, thus disturbing said signals and therefore interfering with the detection of a cycle period. In addition, when the tracking sensor signal is supplied directly to the tracking servo, the sensor signal amplitude is lowered thus varying the gain and therefore making the tracking unstable.
In order to prevent this problem, a method is disclosed in the Japanese Patent Laid Open No. 58-41447 (1983), which uses a peak detection circuit to prevent variation in the reproduced signal caused by the recorded pits. However, the recording pit intervals tend to be reduced due to recent high density recording. In this case, despite the use of the peak detection circuit, the optical frequency characteristic of the information signal decreases the level of the information signal resulting in decreases in the tracking and RF signal levels.
In order to solve the above problem, as shown in, for example, the Japanese Patent Laid-Open No. 62-26647 (1987), an emphasizing circuit is provided which compensates for the decrease in the frequency response of the information signal, and a method is provided for flattening the frequency characteristic by the emphasizing circuit so as to detect the peak. This method eliminates the amplitude reduction of the tracking sensor signal and the RF signal, operates the tracking servo in a stable manner so that it is not modulated by the information signal, and can properly detect the track traverse, thereby being effective in stabilization of access control.
The preceding example, however, requires the same processing for both the tracking servo sensor signal and the access track count detector signal, whereby the frequency characteristic must be accurately compensated for. Accurate processing is required because in order to stably operate the tracking servo, the gain thereof needs to be kept constant, because there is a fear that when the gain of the servo decreases, the tracking accuracy of the servo diminishes and when the gain of the servo rises, the servo may oscillate. Since the gain of the tracking servo changes in proportion to the amplitude of the tracking sensor signal, any variations in the sensor signal amplitude must be completely restrained and therefore, the frequency characteristic must be compensated for accurately. Since the frequency characteristic changes as a function of the radius of the disc, the emphasizing circuit must be fine-tunable at various states and therefore results in a complicated circuit design.